1. Field of the Invention
This invention relates to dental materials made from calcium phosphate glass-ceramics having the color tone equivalent or close to that of natural teeth. The dental materials in this invention mean the materials used for crown inlays, bridges, etc. (hereinafter referred to simply as tooth-crown) and artificial teeth combining tooth-roots, implants, etc. (hereinafter called simply tooth-root) with the tooth-crowns.
2. Prior Art
Materials used conventionally for dental restoration of tooth-crowns, tooth-roots, etc. have been metals and plastics. However, these materials have the following defects. That is, they have seriously poor affinity with the living body, while such affinity is one of the most important characteristics required for medical material. Furthermore, depending on the condition of their use, metals or plastics are eluted, thus causing harmful effects to the living body. Recently, with the purpose of solving these problems, attempts are being made to use ceramic materials, such as alumina, for the foregoing dental materials. However, these ceramic materials are better than the former only in that they are not harmful to the living body, and they are also low in affinity to the living body.
As the materials which are free of the above-mentioned defects of various types of materials, calcium phosphate glass-ceramics are offered. These calcium phosphate glass-ceramic materials are ideal as dental materials in light of their characteristics as mentioned below. (1) As are natural teeth, these materials are composed primarily of phosphorus and calcium, and have a high affinity with gingival tissue. (2) Through crystallization, their strength is improved. Besides their hardness is almost the same as that of natural teeth. Therefore, mutual wear during chewing can be lowered (restricted). (3) During use, elution of the material components due to saliva, food, etc. is not caused, and even if it is caused, the eluted material components do not affect the human body at all because they are identical with the components of natural teeth. (4) Their melting points are similar to those of metallic materials. Accordingly, they can be cast by the lost wax process.
The disadvantages of such excellent calcium phosphate glass-ceramics are the whitish color tone and the inferior aesthetic appearance resulting from the difference in color tone from that of natural teeth.
In order to improve the aesthetic appearance, for metallic materials, facing processes are employed. These facing processes include the resin facing process to cover with acrylic hard resin, and the porcelain facing process to enamel with ceramic materials through baking. All of these processes, however, have the same shortcoming as that caused when the entire portion of artificial teeth-crown are made of resin or porcelain. Also, in the porcelain facing, when the aesthetic effect is particularly required, coating and baking processes must be repeated for three layers, i.e., the opaque layer, the dentin layer and the enamel layer. Accordingly, the time and cost for this process becomes enormous.
Therefore, it is preferable to color the starting materials themselves by adding coloring agents to the starting materials. However, the addition of pigments presents the possible danger that such pigments may harm the living body by eluting gradually during the use of the artificial teeth in which they are contained.